Dental ovens which fire dental restoration parts in firing cycles have been known for a long time. Thus, DE 1 160 777 A discloses an electrically heatable dental oven comprising a negative pressure source in the form of a piston pump as a negative pressure pump. As the pump, depending on its design, causes a significant level of operating noise, it is preferably operated via a negative pressure or suction line which is several meters long, i.e., it is spatially separated slightly from the dental oven. Hereby, the suction line is appropriately formed as a hose which is resistant to negative pressure and flexible to a certain extent. In addition, this hose must be resistant to temperatures as the air which is released by the firing chamber of the dental oven may be relatively hot, although it is being cooled when passing through the wall of the oven.
Typically, thermal stability of up to 300° C. must be given, in fact as a first approximation independently of the fact if the dental oven is operated at a firing temperature of e.g., 1200° C. or at a relatively high firing temperature of 1600° C.
Recently, “muffle ovens” have been used as dental ovens for the production of dental restoration parts. An example of this is disclosed in DE 40 02 358 C1. In such dental ovens, the ceramic part to be shaped/produced is introduced as a blank or “green body” into a sprue which is formed in a prepared muffle made of gypsum. The muffle is heated together with the dental restoration part, in fact, in most cases, after it has already been heated to a starting temperature of e.g., 700° C. in a “pre-heating furnace”. The firing chamber of this dental oven is put under negative pressure by a negative pressure pump which produces negative pressure, i.e., a so-called “vacuum”. Subsequently, a press plunger presses the blank and upon softening of the dental restoration material due to the increasing temperature, the restoration material, favored by the negative pressure which exists in the entire firing chamber, reaches the mold cavities in a pore-free manner whose shape corresponds to that of the dental restoration parts to be produced.
This method has been known for a long time and has stood the test of time, whereas, of course, a condition for the successful provision of the dental restoration parts is that enough negative pressure exists in the entire firing chamber, i.e., also in the muffle itself.
Gypsum or similar casting compounds are not gas-tight, nor liquid-tight, but have a certain degree of flow resistance so that the formation of a completely balanced negative pressure in the firing chamber requires an operation of the suction pump, which forms the negative pressure source, of at least several seconds or minutes. Especially with pre-heated muffles, the “starting temperature” is then already significantly higher than the room temperature, and hot air flows into the suction line immediately when the suction pump is turned on.
In order to prevent the suction pump from being damaged by the hot air, which may also be contaminated, the manufacturers at least of the pumps, or maybe also the manufacturers of the dental ovens, stipulate that the negative pressure line must not be shorter than a certain length, e.g., 3 meters. In order to ensure the desired flexibility of the negative pressure line, the inner diameter, thus the outer diameter, is limited to measurements which are favorable for production and which also make possible the use of standard negative pressure lines. The length of such negative pressure lines can also be adapted to the spacial circumstances in the dental laboratory, as they are available in the piece.
A further advantage of the slightly longer suction line is that a certain “pressure elasticity” is provided; the negative pressure line acts as a buffer and at the same time prevents negative pressure peaks which can be introduced by the negative pressure pump depending on its construction.
A serious disadvantage which up to now, however, prevented the desired design of a negative pressure line in any desired length was and is the tendency of these lines to clog. Due to the large temperature gradient between the firing chamber and the ambient air, the humidity from the firing chamber, which is not carried along, condenses in the negative pressure line. This not only leads to an increase in flow resistance but in particular also to a worsening of the quality of the negative pressure, as the evaporation temperature of the water droplets in the negative pressure line decreases as the negative pressure increases.
In view of this fact, the manufacturers of dental ovens have regularly realized special dehumidification programs which make various attempts to remove the water droplets located in the negative pressure lines.
It has also been suggested to provide several negative pressure lines without further ado and to simply exchange them when the formation of droplets is too high. But this is impractical and requires additional storage, and it may also result in sealing problems when re-assembling the fresh negative pressure line.
It has been known for a longer period of time that the muffle itself and/or the dental restoration material which consists of dental ceramics can be exposed to a considerable degree of residual humidity.
In order to remove the residual humidity, the dental oven is operated at a dehumidification temperature under negative pressure. During this dehumidification program the valves of the dental oven are opened towards the external negative pressure source so that air which is enriched with water vapor flows through the suction line to the negative pressure source. By doing this, the suction line is excessively soaked. This can be problematic for the dental ceramics which are subsequently heated and for the negative pressure, which must be maintained in the desired quality during the pressing process.
When the deposition of moisture is too high in the long suction line, it does not function anymore, which has a negative impact on the quality of the restoration.